Products made from elastomer material, such as tires and the like, are usually formed within a mold, in which they are kept until the completion of the cross-linking process of the material which is brought from an initial fluid or paste-like state into the solid state characteristic of the finished product by such cross-linking.
The cross-linking, which consists of the formation of transverse links between the chains of the polymer forming the elastomer, takes place over a time dependent upon a number of factors related both to the composition of the mixture and the agents used for the cross-linking and also to external factors, such as pressure and temperature, and it is therefore essential experimentally to establish how the cross-linking of the mixture under examination progresses as time goes by in order to be able to supply precise operating data for a successful molding operation.
It is in fact essential for the item obtained to be kept inside the mold until cross-linking is completed or at least until it has reached a certain stage, enabling the item to keep the required shape, without however unduly prolonging its stay in the mold, thus avoiding a limitation of productivity and preventing excessive cross-linking which would cause an alteration in the properties of the product.
For this purpose and according to a known technique, laboratory tests are carried out on a sample of the mixture out of which the item has to be made, in order to establish the required length of setting time under specific operating conditions and thereby enabling the molding apparatus to be adjusted accordingly.
Vulcanization testers having oscillating rotors are used, among other means, according to agreed standards, the rotor being inserted in a chamber containing the mixture undergoing cross-linking and being kept in oscillating motion within it, and the variation with time of the torque required to maintain constant the oscillation amplitude being measured.
A considerable increase in the measured torque provides an indication that cross-linking is taking place, such cross-linking being completed when the measured torque again beomes constant or varies only slowly, having higher values than those of the initial torque.
The data relating to the variations of the torque over the time during which cross-linking is taking place enables the cross-linking time to be established, under specific conditions, in the moulded product to the required degree.
This method, however, is burdensome as it requires a vulcanization tester equipped with an oscillating rotor, which is a complex instrument with a delicate structure; moreover the condition under which the mixture being tested adheres to the walls of the chamber and to the rotor is critical, and finally production conditions must be kept constant because, should they vary, the data obtained during the tests would lose their value.